ceres-solver: per default disable CUDA support (cpp-pm#665)

The latest ceres-solver `v2.1.0-p1` version tries to build against
CUDA, introducing an unwanted runtime dependency.

The user should explicitly choose to build against system CUDA
libraries (or provided by hunter).

Fixes: cpp-pm#657

cmake/projects/ceres-solver/hunter.cmake

@@ -105,6 +105,8 @@ hunter_cmake_args(ceres-solver CMAKE_ARGS
     LAPACK=OFF
     SUITESPARSE=OFF
     CXSPARSE=OFF # since 1.14.0-p0
+    # user must explicitly opt in to compile wit CUDA support, since v2.1.0-p0
+    CUDA=OFF
     # OpenBLAS flag, alternative to LAPACK since v2.1.0-p0
     WITH_OPENBLAS=OFF
     # don't build tests